diff --git a/sdk-platform-java/gapic-generator-java/pom.xml b/sdk-platform-java/gapic-generator-java/pom.xml
index 18b0f8244e48..5d868eba8370 100644
--- a/sdk-platform-java/gapic-generator-java/pom.xml
+++ b/sdk-platform-java/gapic-generator-java/pom.xml
@@ -398,11 +398,6 @@
       <version>${junit.version}</version>
       <scope>test</scope>
     </dependency>
-    <dependency>
-      <groupId>com.google.code.findbugs</groupId>
-      <artifactId>jsr305</artifactId>
-      <version>3.0.2</version>
-    </dependency>
     <dependency>
       <groupId>com.google.api</groupId>
       <artifactId>api-common</artifactId>
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/AnnotationNode.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/AnnotationNode.java
index 7cb0f5ec37fa..514849603cea 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/AnnotationNode.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/AnnotationNode.java
@@ -19,8 +19,10 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class AnnotationNode implements AstNode {
   public static AnnotationNode OVERRIDE =
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ClassDefinition.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ClassDefinition.java
index 1e09e39f4089..4697f284e6ef 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ClassDefinition.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ClassDefinition.java
@@ -21,8 +21,10 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class ClassDefinition implements AstNode {
   // Optional.
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ConcreteReference.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ConcreteReference.java
index b768d5b2ff4f..da97852bf299 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ConcreteReference.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ConcreteReference.java
@@ -19,8 +19,10 @@
 import java.util.Arrays;
 import java.util.List;
 import java.util.Objects;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class ConcreteReference implements Reference {
   private static final String EXTENDS = "extends";
@@ -176,7 +178,7 @@ public boolean isWildcard() {
   }
 
   @Override
-  public boolean equals(Object o) {
+  public boolean equals(@Nullable Object o) {
     if (!(o instanceof ConcreteReference)) {
       return false;
     }
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/JavaDocComment.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/JavaDocComment.java
index 7184044598c2..a5873499ecd9 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/JavaDocComment.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/JavaDocComment.java
@@ -23,7 +23,10 @@
 import java.util.List;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class JavaDocComment implements Comment {
   @Override
@@ -48,11 +51,11 @@ public abstract static class Builder {
     static final String PARAM_INDENT = "       ";
 
     // The lack of a getter for these local variables in the external class is WAI.
-    String throwsType = null;
-    String throwsDescription = null;
-    String deprecated = null;
-    String internalOnly = null;
-    String returnDescription = null;
+    @Nullable String throwsType = null;
+    @Nullable String throwsDescription = null;
+    @Nullable String deprecated = null;
+    @Nullable String internalOnly = null;
+    @Nullable String returnDescription = null;
     List<String> paramsList = new ArrayList<>();
     List<String> componentsList = new ArrayList<>();
 
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/MethodDefinition.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/MethodDefinition.java
index 6bd3bca9c532..1e6bbcd961e6 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/MethodDefinition.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/MethodDefinition.java
@@ -23,8 +23,10 @@
 import java.util.List;
 import java.util.Set;
 import java.util.stream.Collectors;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class MethodDefinition implements AstNode {
   static final Reference RUNTIME_EXCEPTION_REFERENCE =
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/MethodInvocationExpr.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/MethodInvocationExpr.java
index 2a0c2a45c48f..e40856a48707 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/MethodInvocationExpr.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/MethodInvocationExpr.java
@@ -19,8 +19,10 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class MethodInvocationExpr implements Expr {
   public abstract IdentifierNode methodIdentifier();
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/Reference.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/Reference.java
index 4432c68616aa..92732a90de9a 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/Reference.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/Reference.java
@@ -16,8 +16,10 @@
 
 import com.google.common.collect.ImmutableList;
 import java.util.List;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 public interface Reference extends AstNode {
   @Override
   void accept(AstNodeVisitor visitor);
@@ -38,11 +40,9 @@ public interface Reference extends AstNode {
   // The nested types in left-to-right order, if any.
   // Example: com.google.Foo.Bar.Car.ThisType will have the outer types listed in the order
   // [Foo, Bar, Car].
-  @Nullable
-  ImmutableList<String> enclosingClassNames();
+  @Nullable ImmutableList<String> enclosingClassNames();
 
-  @Nullable
-  Reference wildcardUpperBound();
+  @Nullable Reference wildcardUpperBound();
 
   Reference copyAndSetGenerics(List<Reference> generics);
 
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ScopeNode.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ScopeNode.java
index cf4c5326dc21..83249892710f 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ScopeNode.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ScopeNode.java
@@ -14,6 +14,8 @@
 
 package com.google.api.generator.engine.ast;
 
+import org.jspecify.annotations.Nullable;
+
 public class ScopeNode implements AstNode {
   private enum ScopeKind {
     LOCAL, // Placeholder for local scopes.
@@ -54,7 +56,7 @@ public void accept(AstNodeVisitor visitor) {
   }
 
   @Override
-  public boolean equals(Object o) {
+  public boolean equals(@Nullable Object o) {
     if (!(o instanceof ScopeNode)) {
       return false;
     }
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ThrowExpr.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ThrowExpr.java
index 5e4808d28aa9..1b5400ce2cf2 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ThrowExpr.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/ThrowExpr.java
@@ -16,8 +16,10 @@
 
 import com.google.auto.value.AutoValue;
 import com.google.common.base.Preconditions;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class ThrowExpr implements Expr {
   // TODO(miraleung): Refactor with StringObjectValue and possibly with NewObjectExpr.
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/TryCatchStatement.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/TryCatchStatement.java
index 77cd964aed49..6cebf09774c8 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/TryCatchStatement.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/TryCatchStatement.java
@@ -20,9 +20,10 @@
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
-import javax.annotation.Nonnull;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class TryCatchStatement implements Statement {
 
@@ -61,7 +62,7 @@ public abstract static class Builder {
 
     public abstract Builder setIsSampleCode(boolean isSampleCode);
 
-    public Builder addCatch(@Nonnull VariableExpr variableExpr, List<Statement> body) {
+    public Builder addCatch(VariableExpr variableExpr, List<Statement> body) {
       List<VariableExpr> catchVarExprs = new ArrayList<>(catchVariableExprs());
       catchVarExprs.add(variableExpr);
       setCatchVariableExprs(catchVarExprs);
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/TypeNode.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/TypeNode.java
index 12236842fccd..21b5f136ce73 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/TypeNode.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/TypeNode.java
@@ -21,8 +21,10 @@
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Objects;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class TypeNode implements AstNode, Comparable<TypeNode> {
   static final Reference EXCEPTION_REFERENCE = ConcreteReference.withClazz(Exception.class);
@@ -254,7 +256,7 @@ public void accept(AstNodeVisitor visitor) {
 
   // Java overrides.
   @Override
-  public boolean equals(Object o) {
+  public boolean equals(@Nullable Object o) {
     if (!(o instanceof TypeNode)) {
       return false;
     }
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/VaporReference.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/VaporReference.java
index 0c5b976a180c..61a1f11ee491 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/VaporReference.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/VaporReference.java
@@ -20,8 +20,10 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class VaporReference implements Reference {
   private static final String DOT = ".";
@@ -109,7 +111,7 @@ public boolean isWildcard() {
   }
 
   @Override
-  public boolean equals(Object o) {
+  public boolean equals(@Nullable Object o) {
     if (!(o instanceof VaporReference)) {
       return false;
     }
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/VariableExpr.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/VariableExpr.java
index f882c97e8d3b..5abc82fe3226 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/VariableExpr.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/ast/VariableExpr.java
@@ -22,8 +22,10 @@
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.stream.Collectors;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class VariableExpr implements Expr {
   public abstract Variable variable();
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/writer/ImportWriterVisitor.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/writer/ImportWriterVisitor.java
index 1d834043c863..b6e9d23a4ad6 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/writer/ImportWriterVisitor.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/engine/writer/ImportWriterVisitor.java
@@ -69,9 +69,10 @@
 import java.util.Set;
 import java.util.TreeSet;
 import java.util.stream.Collectors;
-import javax.annotation.Nonnull;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 public class ImportWriterVisitor implements AstNodeVisitor {
   private static final String DOT = ".";
   private static final String PKG_JAVA_LANG = "java.lang";
@@ -91,12 +92,12 @@ public void clear() {
     importShortNames.clear();
   }
 
-  public void initialize(@Nonnull String currentPackage) {
+  public void initialize(String currentPackage) {
     this.currentPackage = currentPackage;
     currentClassName = null;
   }
 
-  public void initialize(@Nonnull String currentPackage, @Nonnull String currentClassName) {
+  public void initialize(String currentPackage, String currentClassName) {
     this.currentPackage = currentPackage;
     this.currentClassName = currentClassName;
   }
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/ClientLibraryPackageInfoComposer.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/ClientLibraryPackageInfoComposer.java
index 5bd24bdbd0f0..ff5abc234842 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/ClientLibraryPackageInfoComposer.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/ClientLibraryPackageInfoComposer.java
@@ -32,7 +32,10 @@
 import com.google.common.base.Strings;
 import java.util.logging.Logger;
 import javax.annotation.Generated;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 public class ClientLibraryPackageInfoComposer {
 
   private static final Logger LOGGER =
@@ -47,7 +50,7 @@ public class ClientLibraryPackageInfoComposer {
   private static final String SAMPLE_CODE_HEADER_PATTERN = "Sample for %s:";
   private static final String SERVICE_DESCRIPTION_HEADER_PATTERN = "Service Description: %s";
 
-  public static GapicPackageInfo generatePackageInfo(GapicContext context) {
+  public static @Nullable GapicPackageInfo generatePackageInfo(GapicContext context) {
     if (!context.containsServices()) {
       LOGGER.warning("Generating empty package info since no services were found");
       return null;
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/Composer.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/Composer.java
index 288b02e713f0..edb180676c82 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/Composer.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/Composer.java
@@ -43,7 +43,10 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.stream.Collectors;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 public class Composer {
   public static List<GapicClass> composeServiceClasses(GapicContext context) {
     List<GapicClass> clazzes = new ArrayList<>();
@@ -53,7 +56,7 @@ public static List<GapicClass> composeServiceClasses(GapicContext context) {
     return addApacheLicense(prepareExecutableSamples(clazzes));
   }
 
-  public static GapicPackageInfo composePackageInfo(GapicContext context) {
+  public static @Nullable GapicPackageInfo composePackageInfo(GapicContext context) {
     if (!context.containsServices()) {
       return null;
     }
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/comment/ServiceClientCommentComposer.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/comment/ServiceClientCommentComposer.java
index 64ea4adf7cc5..e3ec116173d5 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/comment/ServiceClientCommentComposer.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/comment/ServiceClientCommentComposer.java
@@ -32,6 +32,7 @@
 import java.util.Map;
 import java.util.Optional;
 import java.util.stream.Collectors;
+import org.jspecify.annotations.Nullable;
 
 public class ServiceClientCommentComposer {
   // Tokens.
@@ -103,7 +104,7 @@ public static List<CommentStatement> createClassHeaderComments(
       String classMethodSampleCode,
       String credentialsSampleCode,
       String endpointSampleCode,
-      String transportSampleCode,
+      @Nullable String transportSampleCode,
       String primaryTransport,
       String secondaryTransport) {
     JavaDocComment.Builder classHeaderJavadocBuilder = JavaDocComment.builder();
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/AbstractServiceStubClassComposer.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/AbstractServiceStubClassComposer.java
index 51e6f410c079..354746e0fbc6 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/AbstractServiceStubClassComposer.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/AbstractServiceStubClassComposer.java
@@ -55,6 +55,7 @@
 import java.util.stream.Collectors;
 import javax.annotation.Generated;
 import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 public abstract class AbstractServiceStubClassComposer implements ClassComposer {
 
@@ -214,7 +215,7 @@ private MethodDefinition createCallableGetterHelper(
   }
 
   private List<MethodDefinition> createOperationsStubGetters(
-      TypeStore typeStore, TypeNode operationsStubType) {
+      TypeStore typeStore, @Nullable TypeNode operationsStubType) {
     List<MethodDefinition> getters = new ArrayList<>();
 
     Iterator<String> operationStubNameIt =
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/AbstractServiceStubSettingsClassComposer.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/AbstractServiceStubSettingsClassComposer.java
index 22cf6b0dfc61..303cf926cf9e 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/AbstractServiceStubSettingsClassComposer.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/AbstractServiceStubSettingsClassComposer.java
@@ -123,8 +123,8 @@
 import java.util.function.Function;
 import java.util.stream.Collectors;
 import javax.annotation.Generated;
-import javax.annotation.Nullable;
 import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 public abstract class AbstractServiceStubSettingsClassComposer implements ClassComposer {
   private static final Statement EMPTY_LINE_STATEMENT = EmptyLineStatement.create();
@@ -1899,8 +1899,8 @@ protected List<MethodDefinition> createNestedClassCreateDefaultMethods(
   protected MethodDefinition createNestedClassCreateDefaultMethod(
       TypeStore typeStore,
       String methodName,
-      String defaultTransportChannelProvider,
-      String defaultTransportChannelProviderBuilder,
+      @Nullable String defaultTransportChannelProvider,
+      @Nullable String defaultTransportChannelProviderBuilder,
       String defaultApiClientHeaderProviderBuilder) {
     List<Statement> bodyStatements = new ArrayList<>();
 
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/AbstractTransportServiceStubClassComposer.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/AbstractTransportServiceStubClassComposer.java
index 6cdf49e38d4c..57bd55cce095 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/AbstractTransportServiceStubClassComposer.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/AbstractTransportServiceStubClassComposer.java
@@ -99,8 +99,8 @@
 import java.util.regex.Pattern;
 import java.util.stream.Collectors;
 import javax.annotation.Generated;
-import javax.annotation.Nullable;
 import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 public abstract class AbstractTransportServiceStubClassComposer implements ClassComposer {
   private static final List<String> AIP_STANDARDS_METHODS =
@@ -912,8 +912,7 @@ protected List<Statement> createLongRunningClient(Service service, TypeStore typ
     return ImmutableList.of();
   }
 
-  @Nullable
-  protected VariableExpr declareLongRunningClient() {
+  protected @Nullable VariableExpr declareLongRunningClient() {
     return null;
   }
 
@@ -1537,8 +1536,7 @@ private static Predicate<String> shouldAutoPopulate(Message methodRequestMessage
    * The Resource Name Extractor should only be generated if the request contains a field that has
    * resource reference (see {@link Field#hasResourceReference()})
    */
-  @Nullable
-  protected LambdaExpr createResourceNameExtractorClassInstance(
+  protected @Nullable LambdaExpr createResourceNameExtractorClassInstance(
       Service service,
       Method method,
       ImmutableMap<String, Message> messageTypes,
@@ -1679,7 +1677,7 @@ protected LambdaExpr createResourceNameExtractorClassInstance(
   }
 
   // Find the first field that has resource reference in a request message
-  private static Field getDestinationResourceIdField(
+  private static @Nullable Field getDestinationResourceIdField(
       Method method, ImmutableMap<String, Message> messageTypes) {
     if (method.inputType().reference() == null
         || method.inputType().reference().fullName() == null) {
@@ -1987,7 +1985,7 @@ private static boolean isAipStandardMethod(List<String> standards, String method
     return standards.stream().anyMatch(standard -> standard.equalsIgnoreCase(methodName));
   }
 
-  private static Set<String> extractLiteralSegments(String pattern) {
+  private static Set<String> extractLiteralSegments(@Nullable String pattern) {
     if (pattern == null || pattern.isEmpty()) {
       return new HashSet<>();
     }
@@ -1999,7 +1997,8 @@ private static Set<String> extractLiteralSegments(String pattern) {
     }
   }
 
-  private static String extractCanonicalResourceName(String pattern, Set<String> knownResources) {
+  private static String extractCanonicalResourceName(
+      @Nullable String pattern, @Nullable Set<String> knownResources) {
     if (pattern == null
         || pattern.isEmpty()
         || knownResources == null
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/TransportContext.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/TransportContext.java
index ab98a86b18b5..d156d3f3b98d 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/TransportContext.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/common/TransportContext.java
@@ -20,8 +20,10 @@
 import com.google.api.generator.gapic.model.Transport;
 import com.google.auto.value.AutoValue;
 import java.util.List;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class TransportContext {
 
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/defaultvalue/DefaultValueComposer.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/defaultvalue/DefaultValueComposer.java
index 3669e9f77e99..bd2c80e28e33 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/defaultvalue/DefaultValueComposer.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/defaultvalue/DefaultValueComposer.java
@@ -54,7 +54,10 @@
 import java.util.Optional;
 import java.util.regex.Pattern;
 import java.util.stream.Collectors;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 public class DefaultValueComposer {
   private static final TypeNode OPERATION_TYPE =
       TypeNode.withReference(ConcreteReference.withClazz(Operation.class));
@@ -120,7 +123,7 @@ public static Expr createValue(
       Map<String, ResourceName> resourceNames,
       Map<String, Message> messageTypes,
       Map<String, String> valuePatterns,
-      HttpBindings bindings) {
+      @Nullable HttpBindings bindings) {
     if (field.isRepeated()) {
       ConcreteReference.Builder refBuilder =
           ConcreteReference.builder().setClazz(field.isMap() ? HashMap.class : ArrayList.class);
@@ -263,7 +266,7 @@ static Expr createResourceHelperValue(
       List<ResourceName> resnames,
       String fieldOrMessageName,
       boolean allowAnonResourceNameClass,
-      HttpBindings bindings) {
+      @Nullable HttpBindings bindings) {
 
     if (isChildType) {
       resourceName = findParentResource(resourceName, resnames).orElse(resourceName);
@@ -547,7 +550,7 @@ public static Expr createSimplePagedResponseValue(
 
   @VisibleForTesting
   static AnonymousClassExpr createAnonymousResourceNameClassValue(
-      String fieldOrMessageName, HttpBindings matchedBindings) {
+      String fieldOrMessageName, @Nullable HttpBindings matchedBindings) {
     TypeNode stringMapType =
         TypeNode.withReference(
             ConcreteReference.builder()
@@ -649,7 +652,7 @@ static AnonymousClassExpr createAnonymousResourceNameClassValue(
         .build();
   }
 
-  public static String constructValueMatchingPattern(String fieldName, String pattern) {
+  public static String constructValueMatchingPattern(String fieldName, @Nullable String pattern) {
     if (pattern == null || pattern.isEmpty()) {
       return fieldName + fieldName.hashCode();
     }
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/grpc/MockServiceImplClassComposer.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/grpc/MockServiceImplClassComposer.java
index d6573f026dae..ae09a6c86a05 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/grpc/MockServiceImplClassComposer.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/composer/grpc/MockServiceImplClassComposer.java
@@ -62,7 +62,10 @@
 import java.util.function.Function;
 import java.util.stream.Collectors;
 import javax.annotation.Generated;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 public class MockServiceImplClassComposer implements ClassComposer {
   private static final MockServiceImplClassComposer INSTANCE = new MockServiceImplClassComposer();
   private static final String IMPL_BASE_PATTERN = "%sImplBase";
@@ -464,7 +467,7 @@ private static MethodDefinition createOnCompletedJavaMethod(
 
   private static Statement createHandleObjectStatement(
       Method protoMethod,
-      VariableExpr requestArgVarExpr,
+      @Nullable VariableExpr requestArgVarExpr,
       VariableExpr responseObserverVarExpr,
       VariableExpr localResponseVarExpr) {
     List<Expr> ifBodyExprs = new ArrayList<>();
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/Field.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/Field.java
index ca2a98b56d27..ebb9240319bc 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/Field.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/Field.java
@@ -19,8 +19,10 @@
 import com.google.auto.value.AutoValue;
 import java.util.Objects;
 import java.util.Optional;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class Field {
   // The field's canonical name, potentially post-processed by conflict resolution logic.
@@ -87,7 +89,7 @@ public boolean canBeAutoPopulated() {
   }
 
   @Override
-  public boolean equals(Object o) {
+  public boolean equals(@Nullable Object o) {
     if (!(o instanceof Field)) {
       return false;
     }
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/GapicBatchingSettings.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/GapicBatchingSettings.java
index 2f7d4de00d84..19214538ad4c 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/GapicBatchingSettings.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/GapicBatchingSettings.java
@@ -17,8 +17,10 @@
 import com.google.auto.value.AutoValue;
 import com.google.common.collect.ImmutableList;
 import java.util.List;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class GapicBatchingSettings {
   public enum FlowControlLimitExceededBehavior {
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/GapicContext.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/GapicContext.java
index 680573bfdb50..dd096e11400f 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/GapicContext.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/GapicContext.java
@@ -25,8 +25,10 @@
 import java.util.Set;
 import java.util.TreeMap;
 import java.util.stream.Collectors;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class GapicContext {
   // Keep a non-AutoValue reference to GapicMetadata, since we need to update
@@ -69,8 +71,7 @@ public GapicMetadata gapicMetadata() {
   @Nullable
   public abstract GapicServiceConfig serviceConfig();
 
-  @Nullable
-  public abstract com.google.api.Service serviceYamlProto();
+  public abstract com.google.api.@Nullable Service serviceYamlProto();
 
   @Nullable
   public abstract String repo();
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/HttpBindings.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/HttpBindings.java
index ae9194eeb2b7..9e37b61f55ef 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/HttpBindings.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/HttpBindings.java
@@ -23,8 +23,10 @@
 import java.util.Map;
 import java.util.Set;
 import java.util.stream.Collectors;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class HttpBindings {
   public enum HttpVerb {
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/LongrunningOperation.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/LongrunningOperation.java
index e23e8a6ee3f8..70e259c186ca 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/LongrunningOperation.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/LongrunningOperation.java
@@ -16,8 +16,10 @@
 
 import com.google.api.generator.engine.ast.TypeNode;
 import com.google.auto.value.AutoValue;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class LongrunningOperation {
   public abstract TypeNode responseType();
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/Message.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/Message.java
index 2866e8462410..664c6f888508 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/Message.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/Message.java
@@ -31,8 +31,10 @@
 import java.util.Map;
 import java.util.stream.Collectors;
 import java.util.stream.IntStream;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class Message {
 
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/Method.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/Method.java
index 66acd065a3f8..45d8dabfaf98 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/Method.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/Method.java
@@ -19,8 +19,10 @@
 import com.google.common.collect.ImmutableList;
 import java.util.ArrayList;
 import java.util.List;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class Method {
   public enum Stream {
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/MethodArgument.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/MethodArgument.java
index e13deda713c8..e1bde1392246 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/MethodArgument.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/MethodArgument.java
@@ -18,6 +18,7 @@
 import com.google.auto.value.AutoValue;
 import com.google.common.collect.ImmutableList;
 import java.util.List;
+import org.jspecify.annotations.Nullable;
 
 @AutoValue
 public abstract class MethodArgument implements Comparable<MethodArgument> {
@@ -47,7 +48,7 @@ public int compareTo(MethodArgument other) {
   }
 
   @Override
-  public boolean equals(Object o) {
+  public boolean equals(@Nullable Object o) {
     if (!(o instanceof MethodArgument)) {
       return false;
     }
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/ResourceName.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/ResourceName.java
index 9a01bf6dba08..4c27b376fd62 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/ResourceName.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/ResourceName.java
@@ -26,8 +26,10 @@
 import com.google.common.collect.ImmutableMap;
 import java.util.List;
 import java.util.Objects;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class ResourceName {
   static final String SLASH = "/";
@@ -128,7 +130,7 @@ public static ResourceName createWildcard(String resourceTypeString, String pakk
   }
 
   @Override
-  public boolean equals(Object o) {
+  public boolean equals(@Nullable Object o) {
     if (!(o instanceof ResourceName)) {
       return false;
     }
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/ResourceReference.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/ResourceReference.java
index 8140a8c2318d..cb996488d085 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/ResourceReference.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/ResourceReference.java
@@ -16,6 +16,7 @@
 
 import com.google.api.generator.gapic.utils.ResourceNameConstants;
 import com.google.auto.value.AutoValue;
+import org.jspecify.annotations.Nullable;
 
 @AutoValue
 public abstract class ResourceReference {
@@ -29,7 +30,7 @@ public boolean isOnlyWildcard() {
   }
 
   @Override
-  public boolean equals(Object o) {
+  public boolean equals(@Nullable Object o) {
     if (!(o instanceof ResourceReference)) {
       return false;
     }
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/Service.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/Service.java
index 656bf6bbd3af..835f086ed5e8 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/Service.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/Service.java
@@ -21,14 +21,15 @@
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Iterables;
 import java.util.List;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 @AutoValue
 public abstract class Service {
   public abstract String name();
 
-  @Nullable
-  public abstract String apiVersion();
+  public abstract @Nullable String apiVersion();
 
   public abstract String defaultHost();
 
@@ -48,8 +49,7 @@ public abstract class Service {
 
   public abstract ImmutableList<Method> methods();
 
-  @Nullable
-  public abstract String description();
+  public abstract @Nullable String description();
 
   public boolean hasDescription() {
     return !Strings.isNullOrEmpty(description());
@@ -79,7 +79,7 @@ public String packageVersion() {
     return "";
   }
 
-  public Method operationPollingMethod() {
+  public @Nullable Method operationPollingMethod() {
     for (Method method : methods()) {
       if (method.isOperationPollingMethod()) {
         return method;
@@ -88,7 +88,7 @@ public Method operationPollingMethod() {
     return null;
   }
 
-  public TypeNode operationServiceStubType() {
+  public @Nullable TypeNode operationServiceStubType() {
     for (Method method : methods()) {
       if (method.hasLro() && method.lro().operationServiceStubType() != null) {
         // All methods within the same service must have the same operationServiceTypeName if
@@ -99,7 +99,7 @@ public TypeNode operationServiceStubType() {
     return null;
   }
 
-  public TypeNode operationType() {
+  public @Nullable TypeNode operationType() {
     for (Method method : methods()) {
       if (method.hasLro() && method.lro().operationServiceStubType() != null) {
         return method.outputType();
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/SourceCodeInfoLocation.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/SourceCodeInfoLocation.java
index f39cb3953066..2256716d1de3 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/SourceCodeInfoLocation.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/model/SourceCodeInfoLocation.java
@@ -15,20 +15,21 @@
 package com.google.api.generator.gapic.model;
 
 import com.google.protobuf.DescriptorProtos.SourceCodeInfo.Location;
-import javax.annotation.Nonnull;
+import org.jspecify.annotations.NullMarked;
 
 /**
  * A light wrapper around SourceCodeInfo.Location to provide cleaner protobuf comments. Please see
  * additional documentation on descriptor.proto.
  */
+@NullMarked
 public class SourceCodeInfoLocation {
-  @Nonnull private final Location location;
+  private final Location location;
 
   private SourceCodeInfoLocation(Location location) {
     this.location = location;
   }
 
-  public static SourceCodeInfoLocation create(@Nonnull Location location) {
+  public static SourceCodeInfoLocation create(Location location) {
     return new SourceCodeInfoLocation(location);
   }
 
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/HttpRuleParser.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/HttpRuleParser.java
index 0ea1c5aa456e..ab35b7afbc2d 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/HttpRuleParser.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/HttpRuleParser.java
@@ -40,13 +40,16 @@
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 import java.util.stream.Collectors;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 public class HttpRuleParser {
   private static final String ASTERISK = "*";
   private static final Pattern TEMPLATE_VALS_PATTERN =
       Pattern.compile("\\{(?<var>[\\w.]*)=(?<template>[^}]+)}");
 
-  public static HttpBindings parse(
+  public static @Nullable HttpBindings parse(
       MethodDescriptor protoMethod, Message inputMessage, Map<String, Message> messageTypes) {
     MethodOptions methodOptions = protoMethod.getOptions();
     if (!methodOptions.hasExtension(AnnotationsProto.http)) {
@@ -131,9 +134,9 @@ private static HttpBindings parseHttpRuleHelper(
 
   private static Set<HttpBinding> validateAndConstructHttpBindings(
       Set<String> paramNames,
-      Message inputMessage,
+      @Nullable Message inputMessage,
       Map<String, Message> messageTypes,
-      Map<String, String> patternSampleValues) {
+      @Nullable Map<String, String> patternSampleValues) {
     ImmutableSortedSet.Builder<HttpBinding> httpBindings = ImmutableSortedSet.naturalOrder();
 
     for (String paramName : paramNames) {
@@ -232,7 +235,7 @@ private static void checkHttpFieldIsValid(String binding, Message inputMessage,
         String.format(messageFormat, field.name(), inputMessage.name(), field.type()));
   }
 
-  private static Map<String, String> constructPathValuePatterns(String pattern) {
+  private static Map<String, String> constructPathValuePatterns(@Nullable String pattern) {
     Map<String, String> varPattern = new HashMap<>();
     if (pattern == null || pattern.isEmpty()) {
       return varPattern;
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/Parser.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/Parser.java
index 234c72cac88a..b8c0fb76e8e7 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/Parser.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/Parser.java
@@ -90,7 +90,10 @@
 import java.util.logging.Logger;
 import java.util.stream.Collectors;
 import java.util.stream.IntStream;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 public class Parser {
   enum SelectiveGapicType {
     // Methods will be generated and exposed externally as usual.
@@ -950,7 +953,7 @@ private static String fetchTypeFullName(String typeName, MethodDescriptor method
   }
 
   @VisibleForTesting
-  static LongrunningOperation parseLro(
+  static @Nullable LongrunningOperation parseLro(
       String servicePackage, MethodDescriptor methodDescriptor, Map<String, Message> messageTypes) {
     MethodOptions methodOptions = methodDescriptor.getOptions();
 
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/ResourceNameParser.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/ResourceNameParser.java
index b03d8aff061a..227dd3c550ec 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/ResourceNameParser.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/ResourceNameParser.java
@@ -35,7 +35,10 @@
 import java.util.Map;
 import java.util.Optional;
 import java.util.Set;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 public class ResourceNameParser {
   /** Returns a map of resource types (strings) to ResourceName POJOs. */
   public static Map<String, ResourceName> parseResourceNames(
@@ -142,7 +145,7 @@ private static Optional<ResourceName> createResourceName(
   }
 
   private static Optional<ResourceName> createResourceName(
-      ResourceDescriptor protoResource, String pakkage, String parentMessageName) {
+      ResourceDescriptor protoResource, String pakkage, @Nullable String parentMessageName) {
     // We may need to modify this list.
     List<String> patterns = new ArrayList<>(protoResource.getPatternList());
     Preconditions.checkState(
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/ResourceReferenceParser.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/ResourceReferenceParser.java
index 06ccf2ec27d6..f16e7a900c0e 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/ResourceReferenceParser.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/ResourceReferenceParser.java
@@ -29,8 +29,10 @@
 import java.util.Map;
 import java.util.Optional;
 import java.util.Set;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 public class ResourceReferenceParser {
   private static final String EMPTY_STRING = "";
   private static final String LEFT_BRACE = "{";
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/RoutingRuleParser.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/RoutingRuleParser.java
index 33c34ff53269..f3cb6007139b 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/RoutingRuleParser.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/RoutingRuleParser.java
@@ -27,10 +27,13 @@
 import com.google.protobuf.Descriptors.MethodDescriptor;
 import java.util.Map;
 import java.util.Set;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 public class RoutingRuleParser {
 
-  public static RoutingHeaderRule parse(
+  public static @Nullable RoutingHeaderRule parse(
       MethodDescriptor protoMethod, Message inputMessage, Map<String, Message> messageTypes) {
     MethodOptions methodOptions = protoMethod.getOptions();
     if (!methodOptions.hasExtension(RoutingProto.routing)) {
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/SourceCodeInfoParser.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/SourceCodeInfoParser.java
index 28ae95c92607..89d669e0b36f 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/SourceCodeInfoParser.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/SourceCodeInfoParser.java
@@ -35,7 +35,8 @@
 import com.google.protobuf.Descriptors.OneofDescriptor;
 import com.google.protobuf.Descriptors.ServiceDescriptor;
 import java.util.Map;
-import javax.annotation.Nullable;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 /**
  * A helper class which provides protocol buffer source info for descriptors.
@@ -48,6 +49,7 @@
  * If you think you need this functionality in a thread-safe context, feel free to suggest a
  * refactor.
  */
+@NullMarked
 public class SourceCodeInfoParser {
   /**
    * A map from file descriptors to the analyzed source info, stored as a multimap from a path of
@@ -60,8 +62,7 @@ public class SourceCodeInfoParser {
   private final Map<Object, String> descriptorToPath = Maps.newHashMap();
 
   /** Gets the location of a message, if available. */
-  @Nullable
-  public SourceCodeInfoLocation getLocation(Descriptor message) {
+  public @Nullable SourceCodeInfoLocation getLocation(Descriptor message) {
     FileDescriptor file = message.getFile();
     if (!file.toProto().hasSourceCodeInfo()) {
       return null;
@@ -70,8 +71,7 @@ public SourceCodeInfoLocation getLocation(Descriptor message) {
   }
 
   /** Gets the location of a field, if available. */
-  @Nullable
-  public SourceCodeInfoLocation getLocation(FieldDescriptor field) {
+  public @Nullable SourceCodeInfoLocation getLocation(FieldDescriptor field) {
     FileDescriptor file = field.getFile();
     if (!file.toProto().hasSourceCodeInfo()) {
       return null;
@@ -81,8 +81,7 @@ public SourceCodeInfoLocation getLocation(FieldDescriptor field) {
   }
 
   /** Gets the location of a service, if available. */
-  @Nullable
-  public SourceCodeInfoLocation getLocation(ServiceDescriptor service) {
+  public @Nullable SourceCodeInfoLocation getLocation(ServiceDescriptor service) {
     FileDescriptor file = service.getFile();
     if (!file.toProto().hasSourceCodeInfo()) {
       return null;
@@ -91,8 +90,7 @@ public SourceCodeInfoLocation getLocation(ServiceDescriptor service) {
   }
 
   /** Gets the location of a method, if available. */
-  @Nullable
-  public SourceCodeInfoLocation getLocation(MethodDescriptor method) {
+  public @Nullable SourceCodeInfoLocation getLocation(MethodDescriptor method) {
     FileDescriptor file = method.getFile();
     if (!file.toProto().hasSourceCodeInfo()) {
       return null;
@@ -101,8 +99,7 @@ public SourceCodeInfoLocation getLocation(MethodDescriptor method) {
   }
 
   /** Gets the location of an enum type, if available. */
-  @Nullable
-  public SourceCodeInfoLocation getLocation(EnumDescriptor enumType) {
+  public @Nullable SourceCodeInfoLocation getLocation(EnumDescriptor enumType) {
     FileDescriptor file = enumType.getFile();
     if (!file.toProto().hasSourceCodeInfo()) {
       return null;
@@ -111,8 +108,7 @@ public SourceCodeInfoLocation getLocation(EnumDescriptor enumType) {
   }
 
   /** Gets the location of an enum value, if available. */
-  @Nullable
-  public SourceCodeInfoLocation getLocation(EnumValueDescriptor enumValue) {
+  public @Nullable SourceCodeInfoLocation getLocation(EnumValueDescriptor enumValue) {
     FileDescriptor file = enumValue.getFile();
     if (!file.toProto().hasSourceCodeInfo()) {
       return null;
@@ -121,8 +117,7 @@ public SourceCodeInfoLocation getLocation(EnumValueDescriptor enumValue) {
   }
 
   /** Gets the location of a oneof, if available. */
-  @Nullable
-  public SourceCodeInfoLocation getLocation(OneofDescriptor oneof) {
+  public @Nullable SourceCodeInfoLocation getLocation(OneofDescriptor oneof) {
     FileDescriptor file = oneof.getFile();
     if (!file.toProto().hasSourceCodeInfo()) {
       return null;
@@ -136,7 +131,7 @@ public SourceCodeInfoLocation getLocation(OneofDescriptor oneof) {
   /**
    * A helper to compute the location based on a file descriptor and a path into that descriptor.
    */
-  private Location getLocation(FileDescriptor file, String path) {
+  private @Nullable Location getLocation(FileDescriptor file, String path) {
     ImmutableList<Location> cands = getCandidateLocations(file, path);
     if (cands != null && cands.isEmpty()) {
       return null;
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/TypeParser.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/TypeParser.java
index 49b17d25dbd0..cb4b8aa313c5 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/TypeParser.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protoparser/TypeParser.java
@@ -34,8 +34,9 @@
 import java.util.Arrays;
 import java.util.List;
 import java.util.Map;
-import javax.annotation.Nonnull;
+import org.jspecify.annotations.NullMarked;
 
+@NullMarked
 public class TypeParser {
   private static Reference REFERENCE_BYTE_STRING = ConcreteReference.withClazz(ByteString.class);
   private static TypeNode TYPE_NODE_BYTE_STRING = TypeNode.withReference(REFERENCE_BYTE_STRING);
@@ -62,7 +63,7 @@ public class TypeParser {
           .put(JavaType.BYTE_STRING, REFERENCE_BYTE_STRING)
           .build();
 
-  public static TypeNode parseType(@Nonnull FieldDescriptor field) {
+  public static TypeNode parseType(FieldDescriptor field) {
     if (field.isMapField()) {
       return createMapType(field);
     }
@@ -83,11 +84,11 @@ public static TypeNode parseType(@Nonnull FieldDescriptor field) {
     return TypeNode.withReference(parseFieldReference(field));
   }
 
-  public static TypeNode parseType(@Nonnull Descriptor messageDescriptor) {
+  public static TypeNode parseType(Descriptor messageDescriptor) {
     return TypeNode.withReference(parseMessageReference(messageDescriptor));
   }
 
-  public static TypeNode parseType(@Nonnull EnumDescriptor enumDescriptor) {
+  public static TypeNode parseType(EnumDescriptor enumDescriptor) {
     return TypeNode.withReference(parseEnumReference(enumDescriptor));
   }
 
@@ -119,7 +120,7 @@ static Reference parseFieldReference(FieldDescriptor field) {
   }
 
   @VisibleForTesting
-  static Reference parseMessageReference(@Nonnull Descriptor messageDescriptor) {
+  static Reference parseMessageReference(Descriptor messageDescriptor) {
     List<String> outerNestedTypeNames = new ArrayList<>();
     FileOptions fileOptions = messageDescriptor.getFile().getOptions();
     String javaOuterClassname =
@@ -179,7 +180,7 @@ static Reference parseMessageReference(@Nonnull Descriptor messageDescriptor) {
   }
 
   @VisibleForTesting
-  static Reference parseEnumReference(@Nonnull EnumDescriptor enumDescriptor) {
+  static Reference parseEnumReference(EnumDescriptor enumDescriptor) {
     // This is similar to parseMessageReference, but we make it a separate method because
     // EnumDescriptor and Descriptor are sibling types.
     FileOptions fileOptions = enumDescriptor.getFile().getOptions();
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protowriter/Writer.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protowriter/Writer.java
index 79c9cbf34912..0f4a892187ad 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protowriter/Writer.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/gapic/protowriter/Writer.java
@@ -34,7 +34,10 @@
 import java.util.List;
 import java.util.jar.JarEntry;
 import java.util.jar.JarOutputStream;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
+@NullMarked
 public class Writer {
 
   static class GapicWriterException extends RuntimeException {
@@ -43,10 +46,10 @@ public GapicWriterException(String errorMessage, Throwable cause) {
     }
   }
 
-  public static final CodeGeneratorResponse EMPTY_RESPONSE = null;
+  public static final @Nullable CodeGeneratorResponse EMPTY_RESPONSE = null;
 
   @VisibleForTesting
-  protected static CodeGeneratorResponse write(
+  protected static @Nullable CodeGeneratorResponse write(
       GapicContext context,
       List<GapicClass> clazzes,
       GapicPackageInfo gapicPackageInfo,
diff --git a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/util/Trie.java b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/util/Trie.java
index 90ff419c52da..b1aeddde7534 100644
--- a/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/util/Trie.java
+++ b/sdk-platform-java/gapic-generator-java/src/main/java/com/google/api/generator/util/Trie.java
@@ -19,13 +19,16 @@
 import java.util.Map;
 import java.util.function.BiFunction;
 import java.util.function.Function;
+import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 /**
  * A common-prefix trie. T represents the type of each "char" in a word (which is a T-typed list).
  */
+@NullMarked
 public class Trie<T> {
   private static class Node<U> {
-    final U chr;
+    final @Nullable U chr;
     // Maintain insertion order to enable deterministic test output.
     Map<U, Node<U>> children = new LinkedHashMap<>();
     boolean isLeaf;
@@ -119,7 +122,7 @@ private <R> R dfsTraverseAndReduce(
     return parentPostprocFn.apply(node.chr, baseValue, leafReducedValue);
   }
 
-  private Node<T> searchNode(List<T> word) {
+  private @Nullable Node<T> searchNode(List<T> word) {
     Map<T, Node<T>> children = root.children;
     Node<T> t = null;
     for (T chr : word) {
diff --git a/sdk-platform-java/gapic-generator-java/src/test/java/com/google/api/generator/gapic/composer/common/BatchingDescriptorComposerTest.java b/sdk-platform-java/gapic-generator-java/src/test/java/com/google/api/generator/gapic/composer/common/BatchingDescriptorComposerTest.java
index ad13e0f45e3b..c3cce3023807 100644
--- a/sdk-platform-java/gapic-generator-java/src/test/java/com/google/api/generator/gapic/composer/common/BatchingDescriptorComposerTest.java
+++ b/sdk-platform-java/gapic-generator-java/src/test/java/com/google/api/generator/gapic/composer/common/BatchingDescriptorComposerTest.java
@@ -48,6 +48,7 @@
 import java.util.Map;
 import java.util.Optional;
 import java.util.Set;
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
@@ -178,7 +179,7 @@ void batchingDescriptor_noSubresponseField() {
     Assert.assertCodeEquals(goldenFilePath, writerVisitor.write());
   }
 
-  private static Method findMethod(Service service, String methodName) {
+  private static @Nullable Method findMethod(Service service, String methodName) {
     return service.methods().stream()
         .filter(m -> m.name().equals(methodName))
         .findFirst()
diff --git a/sdk-platform-java/gapic-generator-java/src/test/java/com/google/api/generator/gapic/composer/common/RetrySettingsComposerTest.java b/sdk-platform-java/gapic-generator-java/src/test/java/com/google/api/generator/gapic/composer/common/RetrySettingsComposerTest.java
index 775a0b1d0932..826e300d58a2 100644
--- a/sdk-platform-java/gapic-generator-java/src/test/java/com/google/api/generator/gapic/composer/common/RetrySettingsComposerTest.java
+++ b/sdk-platform-java/gapic-generator-java/src/test/java/com/google/api/generator/gapic/composer/common/RetrySettingsComposerTest.java
@@ -53,6 +53,7 @@
 import java.util.Optional;
 import java.util.Set;
 import java.util.stream.Collectors;
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
@@ -462,7 +463,7 @@ void batchingSettings_fullFlowControlSettings() {
     assertEquals(expected, writerVisitor.write());
   }
 
-  private static Method findMethod(Service service, String methodName) {
+  private static @Nullable Method findMethod(Service service, String methodName) {
     for (Method m : service.methods()) {
       if (m.name().equals(methodName)) {
         return m;
diff --git a/sdk-platform-java/gapic-generator-java/src/test/java/com/google/api/generator/gapic/model/GapicServiceConfigTest.java b/sdk-platform-java/gapic-generator-java/src/test/java/com/google/api/generator/gapic/model/GapicServiceConfigTest.java
index b42e7e91346c..30cc178a423f 100644
--- a/sdk-platform-java/gapic-generator-java/src/test/java/com/google/api/generator/gapic/model/GapicServiceConfigTest.java
+++ b/sdk-platform-java/gapic-generator-java/src/test/java/com/google/api/generator/gapic/model/GapicServiceConfigTest.java
@@ -34,6 +34,7 @@
 import java.util.Map;
 import java.util.Optional;
 import java.util.Set;
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.Test;
 
 class GapicServiceConfigTest {
@@ -249,7 +250,7 @@ private static Service parseService(FileDescriptor fileDescriptor) {
     return services.get(0);
   }
 
-  private static Method findMethod(Service service, String methodName) {
+  private static @Nullable Method findMethod(Service service, String methodName) {
     for (Method m : service.methods()) {
       if (m.name().equals(methodName)) {
         return m;
diff --git a/sdk-platform-java/gapic-generator-java/src/test/java/com/google/api/generator/gapic/protoparser/SourceCodeInfoParserTest.java b/sdk-platform-java/gapic-generator-java/src/test/java/com/google/api/generator/gapic/protoparser/SourceCodeInfoParserTest.java
index c9e40a8274f2..5c68b288a163 100644
--- a/sdk-platform-java/gapic-generator-java/src/test/java/com/google/api/generator/gapic/protoparser/SourceCodeInfoParserTest.java
+++ b/sdk-platform-java/gapic-generator-java/src/test/java/com/google/api/generator/gapic/protoparser/SourceCodeInfoParserTest.java
@@ -32,6 +32,7 @@
 import java.nio.file.Paths;
 import java.util.ArrayList;
 import java.util.List;
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
@@ -154,7 +155,7 @@ void getOnoeofInfo() {
    *
    * @return the top level target protoFile descriptor
    */
-  private static FileDescriptor buildFileDescriptor() throws Exception {
+  private static @Nullable FileDescriptor buildFileDescriptor() throws Exception {
     try (InputStream in = newDescriptorSetInputStream()) {
       List<FileDescriptorProto> protoFileList = FileDescriptorSet.parseFrom(in).getFileList();
       List<FileDescriptor> deps = new ArrayList<>();